1. Field of the Invention
The present invention relates to the field of two-flow jet engines comprising an elongate secondary flow duct. It relates to the engines of this type which are secured to the fuselage of the aircraft or to military engines.
2. Description of the Related Art
A two-flow jet engine comprises a fan which, when it is at the front of the engine, provides a flow of compressed air which is separated into two concentric annular flows, i.e. a primary flow and a secondary flow which surrounds the primary flow. The latter is guided towards the gas generator part of the engine, which comprises compression stages, a combustion chamber, and a turbine section by means of which the fan is driven. The primary flow containing the combustion gases is then ejected into an exhaust pipe. The secondary flow is rectified downstream from the fan, and is itself ejected. In civilian engines it provides the substantial part of the thrust.
According to one configuration, the secondary flow is guided in a by-pass duct which extends around the engine between the fan and the exhaust pipe of the primary flow, and comprises two coaxial, substantially cylindrical walls which delimit an annular space between one another. The inner wall of the by-pass duct forms the envelope of the gas generator. The outer wall of the by-pass duct forms a duct which extends from the plane of the rectifier fins as far as the level of the exhaust pipe. The outer duct of the by-pass duct is designated in the field by the acronym OFD (outer fan duct).
The engine can be fitted under the wing of the aircraft or along its fuselage, and in particular towards the rear. In this case, the engine comprises a by-pass duct as previously described. The attachments of the engine to the aircraft are situated at the level of two transverse planes, i.e. an upstream plane which passes via the upstream structural casing, designated as the intermediate casing, and a downstream plane which passes via the downstream structural casing, designated as the exhaust casing.
In the case of fitting onto the fuselage, in order to assure the downstream securing, a structural ring is provided on the outer duct of the by-pass duct, i.e. the OFD, this ring being connected by arms or connecting rods to the ferrule or outer ring of the exhaust casing. Patent application EP 2022973 in the name of the applicant describes an example of the structure of the by-pass duct outer duct.
The link between the two above-described rings can be in the form of radial arms which are distributed all around the axis of the engine, and are secured rigidly to the two rings. The link can also be in the form of connecting rods which are inclined relative to the axis of the engine. The connecting rods are secured to the two rings by attachments of the clevis and pin type. An attachment of this type is formed by two, single or double clevises, which are integral, one of them with the end of the connecting rod, and the other with the wall of the ring, and a common pin passes through them.
More particularly, the connecting rods are arranged in pairs, the connecting rods of each pair being tangent to the ring of the exhaust casing, whilst being convergent on an attachment of the ring of the outer duct of the by-pass duct.
Whether the link is formed by radial arms or connecting rods, it is hyperstatic; the forces thus pass via all the arms or connecting rods. In the solutions according to the prior art, all the elements of the link, i.e. pins, clevises, connecting rods or arms, have dimensions such as to withstand mechanically the forces which would be derived from the imbalance generated by the loss of a vane in the engine. The objective is to avoid the risk of the engine stalling if such a critical situation were to arise. The weight of the assembly which forms the link is consequently heavy. In addition, since the forces which can result from breakage of a vane are potentially transmitted to all the arms or connecting rods, the ring of the outer duct of the by-pass duct must also be able to withstand these loads around its entire circumference. Its size is therefore designed accordingly.